Manganese critical levels for soybean growth and physiological processes

The influence of Mn nutrition on soybean (Glycine max (L.) Merr.) dry matter production and physiological processes was determined on plants grown in the greenhouse. Manganese critical deficiency levels for dry matter production under greenhouse conditions were compared to the critical deficiency levels for seed yield under field conditions. The critical deficiency and toxicity levels for dry matter production were developed by relating the blade Mn concentration in recently matured blades to the total dry matter of plants provided with added Mn to produce a growth range from severe deficiency through normal to severe toxicity. The Mn critical deficiency and toxicity levels are defined as the Mn concentration in blades associated with a 10% reduction in dry matter production due to deficiency or toxicity, respectively. The Mn critical levels for dry matter production in ‘Bragg’ soybean were determined as 10 and 160 μg/g in recently matured blades for the deficiency and toxicity levels, respectively. From extensive field experiments on a soil low in available Mn treated with Mn application rates of 0 to 56 kg/ha, the Mn critical deficiency level related to seed yield was 10 μg/g in recently matured blades sampled at the R2 (early bloom) growth stage. The Mn critical deficiency levels for both photosynthesis and total chlorophyll were 10 μg/g in recently matured blades. Respiration and transpiration on a leaf area basis were not influenced by Mn status; therefore, critical deficiency levels could not be determined. Carbonic anhydrase activity was increased 3 fold due to severe Mn deficiency, and, at approximately 9 μg Mn/g in leaf blades, carbonic anhydrase activity was at levels associated with normal appearing plants.